Air displacement pipetters with disposable pipette tips have been used in the medical and laboratory industries for many years. The main reason for such continual acceptance comes from the fact that after each use the pipette tip has traditionally been disposed of thereby limiting the possibility of cross contamination between samples. However as tests become more critical and the need to perform many tests from a limited amount of sample quantity became important, laboratory technicians have begun to have problems. These problems or errors could be contributed to operator use or fatigue, which often causes splashing of the sample. The sample could also aerosol during aspiration of the fluid, or the fluids contaminated gases can flow through the pipette tip upward into the calibrated barrel in the form of air borne contaminates. Even the smallest amount of dispensing error causing volume discrepancy or particles left behind on the barrel of the pipetter from previous tests can invalidate, or skew the evaluations of new test samples causing hours or even days of laboratory research to be wasted.
A researcher's work requires a high degree of volume consistency between samples and when hundreds of filtered or unfiltered pipette tips are used in just one procedure or test, the work may be invalid because of the inaccuracies in these sample volumes due to the precision and accuracy of the volumes dispensed. This is sometimes due to operator fatigue because of the excessive amount of force required to install and replace the disposable pipette tips insuring that a hermetic seal is made between the pipetter barrel and the pipette tip. This is very difficult because if the user does not provide enough downward force, the pipette tip seal may have a leak path which can cause volume discrepancies.
Usually, in mounting a pipette tip on a mounting shaft or barrel of a pipetter, a user, exerting a downward force of between twelve to eighteen pounds, drives the pipetter barrel axially into the upper portion of the pipette tip a distance which to the user seems sufficient to create a air tight seal and a stable non-rocking axial position. On occasion, in a mistaken attempt to improve the seal or axial position, a user will exert a downward insertion force up to twenty-five pounds. Since most pipette tips are formed of a relatively rigid plastic material such as polypropylene, the annular stretching of the pipette tip required to accommodate movement of the pipette tip onto the pipetter barrel is minimal. The inner surface and side walls of the proximal portions of most pipette tips are axially tapered at a one to one and a half degrees and mate with the distal end of the pipetter barrel to form an axially elongated frustoconical annular sealing band. The sealing band is dimensioned to stretch outwardly (“hoop stretch”) as the distal end of the elongated generally shaped conical pipetter barrel mounting shaft is forced into the proximal end of the pipette tip to firmly seat the tip on the barrel and to create an axially elongated annular air tight seal between the sealing band and the pipetter barrel—hopefully maintaining a non-rocking stable axial position. In some instances, contact occurs between the pipetter's ejector sleeve and the upper top surface of the entrance to the pipette tip. This contact, depending on the amount of force and the angle of insertion by the user is not consistent and only provides minimal increase, if any, to the stability and lateral support of the pipette tip in maintaining its axial perpendicularity to the sealing band.
The more firmly a pipette tip is mounted or wedged onto the barrel of the pipetter, the greater the axial force which a pipetter user must generate by thumb and hand action to eject the pipette tip from the barrel when a tip replacement is desired. In practice, it is not uncommon for axial forces exceeding fifteen pounds to be generated by the pipetter users thumb and hand in driving a pipette tip from a mounting shaft. Over several and repeated ejection operations, particularly with multi-channel pipetters where substantially greater axial ejection forces are required, the thumb and hand of the user become physically stressed often resulting in repetitive stress injury to the thumb and hand and in extreme cases, carpal tunnel syndrome.
Because of the length of the sealing region and the relatively thick sidewall of the standard pipette tip, large plastic forces in the tip material resist such outward hoop stretching and require exertions of large axial forces to mount these standard pipette tips onto the pipetter barrels and create the necessary air and fluid type seal. In an effort to reduce the hand and finger forces which pipetter users must generate, some manufactures have reduced the annular sealing region of the sidewall of the pipette tip form 0.030-0.040 inch (0.75-1.00 mm) to 0.008 to 0.020 inch (0.20 to 0.50 mm) trying to provide a more resilient surface with minimal success. While others have decreased the amount of interference between the outer diameter of the pipetter barrel and the inner diameter of the mating sealing zone of the pipette tip to 0.003 inch (0.075 mm) or about the width of a human hair as described in U.S. Pat. No. 6,168,761. As one can imagine, the less interference between these two matting surfaces will reduce the overall axial force necessary to install and eject the pipette tip from the pipetter but at what price. To maintain consistent sealing of two manufactured parts, the pipette tip being of plastic origin, and the barrel sometimes molded plastic or manufactured from stainless steel is very difficult and requires extremely tight manufacturing tolerances below +/−0.001 inch (+/−0.025 mm). The fact is that most pipette tips are manufactured from polypropylene which has one of the higher shrink rates of plastic materials makes this even more difficult to achieve.
When plastic parts are manufactured, a tool is created and the allowance of mold shrinkage must be made. Mold shrinkage of thermoplastic materials is very complex because it is affected by so many factors. For polypropylene, a major factor is cooling rate. Generally, higher shrinkage's result from slower cooling rates, which is why thicker parts shrink more than thinner parts. It simply takes longer to remove heat from thicker parts. Thus, the golden rule for plastic part design is to maintain consistent wall thickness or wall sections unlike that of the new pipette tips mentioned above which reduce the wall sections for example from 0.040 to 0.010 in sealing areas to help reduce the hoop stress but can causes other potential problems such as dimensional stability.
Further, polypropylene will continue to shrink and crystallize for several days or weeks after molding. Studies of shrinkage versus time show that most of the shrinkage takes place within the first 24 hours after molding. A small amount of additional shrinkage will occur in the next 24 hours, followed by incremental amounts (that are difficult to measure) for two to four weeks. Mold shrinkage for polypropylene can vary from about 0.010 inch/inch. (0.025 mm/mm) to about 0.030 inch/inch. (0.750 mm/mm), depending on part thickness, formulation, and processing conditions.
Exposure of molded parts to different temperatures and humidity can also cause thermal expansion or contraction resulting in additional part dimensional changes that can occur during shipment of parts by truck, train or air freight. Still further, climatic changes within different parts of the world or within laboratories can also change the dimensional characteristic of these plastic parts and cause unknown changes that can result in leakage between parts causing volume discrepancy which can invalidate, or skew the evaluations of new test samples causing hours or even days of laboratory research to be wasted. Plastics, unlike metal parts are very sensitive to temperature variations and can grow or shrink depending on the environment that they are in.
In an effort to overcome the dimensional instability of plastic pipette tips and reduce the overall axial forces required to install and eject these parts from pipetters, some manufactures have offered a rubber interface on their pipetter barrels for sealing. For example, the Brinkman Instrument Company has included o-rings on the pipetter barrels of its Transfer-pipette 8/12 to insure the plastic pipette tips maintain air tight seals over dimensional variations between parts while staying firmly mounted during use. The interface concept is good, however, due to the volume of usage of the many plastic pipette tips that are engaged and released again and again over the o-ring surface, the o-ring material begins to wear and thus the plastic pipette tips no longer stay firmly on the pipetter barrels. In addition, the o-rings wear particles can sometimes mix with the sample fluid being transferred and potentially cause contamination to the fluid samples.
Still further, a standard pipette tip as illustrated in U.S. Pat. No. 5,660,797 incorporates a elongated inner collar made from a softer material having a back surface in contact with the inside surface of the pipette tip as shown in FIG. 1 as prior art. The secondary softer material of the elongated sealing collar 80 helps to promote sealing of the pipetter barrel as the elongated collar inner surface 82 surrounds the pipetter barrel by frictional fit as shown and described in FIG. 1 and FIG. 1A of the drawings and application. The large surface area created by the softer inner surface 82 is counter-productive in reducing the axial forces needed to install and eject this pipette tip. The elongated sealing collar 80 surrounds the mating pipetter barrel 48 promoting sealing but increases the frictional interference between the two mating parts creating a high coefficient of friction (the materials do not slide past each other easily). The static or breakaway friction of theses two dissimilar materials in combination with the increased surface area of the mating surfaces requires substantially greater axial forces to eject the pipette tip 79 from the pipetter barrel 48 than prior art configurations. This is especially apparent do to the inherent properties of the softer material to grab the mating pipetter barrel surface unlike the more rigid polypropylene material which has a much lower coefficient of friction. The increased surface contact of the softer material can promote better sealing as was the intention of the invention but the combination of increased surface area of the soft material and no limitation to the axial depth of penetration of the pipetter barrel into the pipette tip can cause more difficulty and increased axial forces in removing the pipette tip from the pipetter than prior art tip configurations. Therefore, this causes thes thumb and hand of the user to become physically stressed which can often resulting in repetitive stress injury. Lastly the structure of the above mentioned pipette tip also does not provide lateral mounting stability unless in those rare instances that the pipette tip is jammed upward against the bottom of the pipetter ejector sleeve providing minimal lateral stability of the pipette tip onto the barrel which there is no description thereof.
While some of the noted pipette tips above have shown some improvement with respect to the axial forces required to install and eject the pipette tip from the pipetter barrel, some have not. The reduced wall sections and the minimal interference fit between the pipetter barrel and the mating annular sealing region are not consistently reliable over a broad range of temperature variations and climates. The use of rubber-like materials to increased the sealing capability of the pipette tip is an improvement over the prior art for sealing, however, the addition sealing capability does not assure the reduction in the axial forces require to mount and eject the pipette tip from the pipetter.
Accordingly, there is a need for an improved ergonomically designed disposable pipette tip which will easily and stably mount onto a pipetter barrel mounting shaft and subsequently be ejected by a substantially reduced pipetter tip ejection force than existing standard disposable pipette tips in the market place today.
For a better understanding of the invention and how this new ergonomic pipette tip overcomes these disadvantages, reference is made to the following Summary, Description of Drawings and the Detailed Description of Invention.